This invention relates generally to an improved alignment fixture and related method of use for taking X-ray images of a selected anatomical region of a patient, such as the pelvis and hip region preparatory to hip arthroplasty or hip replacement surgery, wherein the fixture aligns the pelvis in the X-ray beam and a ruled grid appears in the X-ray image to overlie the anatomical region of interest with a known parallax magnification corresponding with the parallax magnification of the anatomical region. Accordingly, X-ray images taken with the improved alignment fixture consistently align the pelvis and the projected grid and can be scaled quickly and easily for use in selecting a prosthesis size suitable for the specific patient.
X-ray imaging is a known medical technique wherein X-rays are generated from a beam source to pass through a selected anatomical region of a patient. Some of the X-rays are absorbed by the patient whereas others pass through the patient to impact a sheet of X-ray film or recording media, thereby producing a shadow-type image of the anatomical region of interest. In this regard, in many X-ray systems, the X-rays are generated from an essentially point source and travel downwardly through the selected anatomical region of the patient to impact the X-ray film or recording media underlying the patient. This X-ray imaging arrangement inherently produces magnification of the selected anatomical region by parallax, due to the fact that the X-ray beam expands as it travels from the point source, and the anatomical region is not located in the same plane as the X-ray film or recording media. Accordingly, the actual X-ray image does not correspond exactly with the size of the patient's anatomical structure. While this parallax magnification is not a problem in some X-ray imaging applications, such as diagnosis of a bone fracture or the like, it can be particularly problematic in an X-ray image taken prior to a surgical procedure involving implantation of a prosthesis, such as arthroplastic hip replacement surgery, wherein a primary reason for taking the pre-surgery X-ray image is to enable the surgeon to plan the surgery and to select a prosthesis of suitable size and design for the specific anatomy of the surgical patient.
A variety of devices and systems have been developed in the prior art in an attempt to accommodate or compensate for parallax image magnification in an X-ray image taken preparatory to a prosthesis implantation surgery. Such prior art devices and systems generally relate to placement of a ruled grid in overlying relation with the patient anatomy, so that the ruled grid can provide an indication of actual anatomical size. However, the ruled grid cannot be physically located at the same vertical plane as the patient's anatomical region, particularly when that anatomical region comprises a structure such as a hip joint located deep within the patient. For this reason, such prior art devices and systems have generally required undue estimation when used to indicate the size of a deep anatomical structure, such as a hip joint. Accordingly, such prior art devices and systems have relied for accuracy largely upon the technique and skill of an individual X-ray technician, whereby the actual placement of the ruled grid in relation to the anatomical region of interest has been subject to an unacceptable margin of error. As a result, confident pre-surgical prosthesis size determination based upon a pre-surgical X-ray image has remained an elusive objective. Prior art does not address aligning the pelvis in the X-ray beam which is integral with consistent and accurate planning of the hip or pelvis surgery.
There exists, therefore, an on-going and substantial need for an improved X-ray imaging alignment fixture that is relatively simple and easy-to-use, without requiring undue skill by the individual X-ray technician, to permit quick and easy alignment of the pelvis in the X-ray beam and scaling of a resultant X-ray image in relation to a ruled grid, and thereby permit confident selection by an examining surgeon or the like of an appropriately sized prosthesis for a specific patient. The present invention fulfills these needs and provides further related advantages.